Automobiles equipped with automatic transmissions have become ubiquitous. A conventional automatic transmission includes a planetary gearset, a set of bands used to engage parts of the gearset, a set of wet-plate clutches to engage other parts of the gears, a hydraulic system controlling the clutches and bands, and a transmission fluid pump. Numerous solenoid valves or actuators are used to control the flow of transmission fluid through the automatic transmission. The typical solenoid valves that are used in an automatic transmission include variable bleed solenoid (VBS) valves and variable flow solenoid (VFS) valves.
A VBS valve is a current-controlled, electro-hydraulic actuator that provides an outlet pressure that is a precise function of the current applied to the valve. A constant supply pressure is supplied to the valve through a fixed control orifice to a control chamber formed by the valve. The control chamber pressure can be controlled by allowing the control chamber to bleed to a reservoir through the variable orifice formed by the VBS valve. With this structure, a VBS valve can regulate fluid line pressure from a maximum value to a minimum value. However, fluid flow also ranges from a maximum value to a minimum value during operation of the VBS valve, which reduces efficiency of the transmission fluid pump.
VFS valves also regulate fluid line pressure from a maximum value to a minimum value, but they do so using a spool valve which minimizes fluid flow at both maximum and minimum pressure values. Unfortunately, VFS valves cannot effectively minimize fluid leakage through the valve while maximizing movement of the spool valve with spool movement being desirable for promoting good operation of the valve operation. In other words, when leakage is minimized the spool valve undesirably tends to stick, whereas when spool movement is maximized, undesirable leakage tends to increase.
The present invention has recognized these prior art drawbacks, and has provided the below-disclosed solutions to one or more of the prior art deficiencies.